Draft producing apparatus



s. KNEASS, JR 2,350,401 DRAFT Pnonucme APPARATUS June 6, 1944.-

Filed Sept. 25, 1941 lo I 5 JNVENTOR v I SJRICKLAND KNEASS,JR. BY QawATTOR NEY Patented June 6, 1944 DRAFT PRODUCING APPARATUS StricklandKneass, Jr., Boylston, Mass, assignor to Morgan Construction Company,Worcester, Mass, a corporation of Massachusetts Application September25, 1941, Serial No. 412,330

7 Claims.

This invention relates to draft producing apparatus, and moreparticularly to the construction of an improved apparatus in the form ofan ejector stack arranged to utilize air under pressure for theentrainment and discharge of hot gases, such as the waste gaseousproducts of combustion from a furnace or the like.

The air is ordinarily supplied to such stacks by means of a power drivenfan, and the power required by the fan is the principal item of expensein the operation of the apparatus.

It is accordingly one object of the invention to provide a draftproducing apparatus particularly adapted for handling hot gases and ofsuch a nature as to require comparatively little power.

It is a further object of the invention to provide an ejector stackarranged to be supplied with ejector air from a fan and so constructedthat the power required to drive the fan will be greatly reduced ascompared with prior apparatus.

With these and other objects in view, as will be apparent to thoseskilled in the art, the invention resides in the combination of partsset forth in the specification and covered by the claims appendedhereto.

Referring to the drawing illustrating one embodiment of the inventionand in which like reference numerals indicate like parts,

Fig. 1 is a vertical section through an ejector stack, the section beingtaken on the line l--l of Fig. 2;

Fig. 2 is a section on the line 2-2 of Fig. 1;

Fig. 3 is a section on the line 33 of Fig. 1;

Fig. 4 is a fragmentary view similar to Fig. 1 but showing amodification; and

Fig. 5 is a section on the line 55 of Fig. 4.

The embodiment illustrated comprises a vertical passage l0 for theupward flow of the hot gaseous products of combustion from a furnace(not shown). In the type of installation for which the invention isparticularly adapted, these gases will ordinarily be at a temperature ofover 1000 degrees Fahrenheit, and in some cases they may be as high as2000 degrees. Above the passage I0 and connected thereto is an uprightmetal stack II with its upper end open to the atmosphere. This stack isconstructed in the form of a Venturi tube, with a throat l2 spaced fromthe lower end of the stack a distance somewhat less than half the heightthereof. Directly below-the throat there is an upwardly convergingportion I4, whereas the portion above the throat diverges gradually inthe upward direction to form a so-called diffuser. An upwardly directedjet of air is discharged into the stack adjacent the throat l2, and forthis purpose a nozzle I6 is provided. Air under pressure is supplied tothis nozzle by means of a rotary fan or blower l8.

It will now be understood that with the construction as so far describedthe air discharged by the nozzle IE will entrain the hot gases in thestack and eject them upwardly to the atmosphere, producing a desireddraft or suction in the passage 10. However, a considerable quantity ofair will be required at an appreciable pressure, so that the powernecessary to drive the fan will be a substantial item of expense. I havediscovered that this power can be very materially reduced bytransferring heat from the hotgases to the air at a sufiicient rate toraise the temperature of the air leaving the nozzle to a comparativelyhigh value. This discovery is substantiated by the following theoreticaldiscussion relating to the behaviour of a jet of air discharged througha nozzle under a pressure head.

Let

P=power available from the jet in foot-pounds per second.

11) =weight of air in pounds per second.

H=head of the air in inches of water.

h =head of the air in feet of air.

T=absolute temperature for standard conditions,

in degrees Fahrenheit (460+).

t =absolute temperature of air entering nozzle, in degrees Fahrenheit.

y =pounds of air per cubic foot at standard conditions (30" Hg and 60degrees F).

g =acceleration due to gravity in feet per second v =velocity of airleaving nozzle in feet per second.

a =area of nozzle in square feet.

Then We have as basic formulae From the known weight of water, and theeffect of temperature on the density of gases, We have From 1) and (4),

From (2) and (5), and the effect of temperature on the density of gases,

From (3), (4) and (6),

Assuming that the head or pressure of the air is constant, there are novariables in Equation 8 except it. Hence 9) I Pam Y where K isaconstant.

Thus the power available from the jet is proportional to the square rootof the absolute temperature of the air entering the nozzle. Hence if theair is heated to 600 F. after compression by the fan, we would have V(10) P= K 600+460=K /1060=32.6K I

If the air is not heated and enters the nozzle at 60 F wewould have Fromthe above it will be seen that the air when heatedto 600 F. will deliver1.43 times as much power through the same nozzle area as the cold air.

Furthermore, theweight of the air discharged will be appreciably lesswhen the air i heated, as will now be shown. In Equation 6 the factorsa, g, H, y and T are all constants. Therefore we canwrite AL: l M/c0+460 /'2'0*2Ts From (13) and (14) we have the ratio 22.8 7 70 Thus byheating the air to 600 after it has been compressed by the fan there isobtained 1.43 times the amount of power from the same nozzle, and theweight of air handled by the fan is only .7 of that when discharging at60. Accordingly from a given weight of air at 600 I will obtain the hotgases which are being ejected. For this purpose the stack H is enclosedby an external casing or jacket to provide an annular passage 2| throughwhich the air may flow downwardly in counterflow heat-transferrelationship to the hot gases. This casing preferably extends for asubstantial distance both above and below the throat I2, to provide along path of travel for the air.

Furthermore, the width of the passage 2| (measured radially) iscomparatively small so that the air velocity will be maintained highenough to ensure a rapid transfer of heat through the metal times asmuch energy as would be obtained from a the same weight of air at 60.

wall of the stack ll. At the upper end of the casing 20 the air passageis enlarged to provide an annular inlet chamber 23 which is connected tothe fan l8 by a duct 24. Similarly, the air passage is enlarged at thelower end of the casing to provide an annular outlet chamber 26 which isconnected to the nozzle l6 by a duct 21. In order to accommodatedifferential expansion between the stack l and the casing 20, an annularwater seal 28 is provided at the top of the casing. In-

the event the stack is to be employedin connection with a reversiblefurnace as a part of the wellknown Isley system, I may mount aslide'damper 30 at the throat l2, as shown in Figs. 4 and 5. In thisconstruction the passage 2| is divided at the throat into upper andlower portions which are connected by three short externalv ducts 3|arranged'to avoid interference with the damper 30.

It will now be apparent that in the operation of the invention the fanl8 will deliver air at atmospheric temperature through th duct 24 to thechamber 23, and this air will flow downwardly through the passage 2| tothe chamber 26, and thence through the duct 21 to the nozzle IS. Thenozzle will discharge the air in an upwardly directed jet, which willentrain the hot gases from the passage l0 and cause them to flowupwardly through the stack II to the atmosphere. In the course of itsdownward flow through the passage 2| the air will be heated toacomparatively high temperature of say 600 F. by the transfer of heatfrom the hot gases. Because of the increased efiiciency of the hot airjet in ejecting the gases, as explainedabove, it is possible to reducethe weight of air discharged as compared with prior ejectors utilizingcold air. Thus the area of the nozzle I6 will be less, the fan |8 willbe smaller, and the motor or other device which drives the fan will beless powerful, thereby greatly decreasing the original cost as well asthe expenditure for operating power. The slide damper 30 of Figs. 4 and5 will of course be open when the hot gases are being ejected, andclosed when the furnace is reversed and the air is to be used forcombustion purposes.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is: I

1. Apparatus for the drafting of hot gases comprising a conduit for theflow of the gases to the atmosphere, a nozzle arranged to discharge airinto the conduit to entrain the gases and cause flow thereof in thedirection desired, means providing a passage through which air may flowand be subjected to heating by the gases flowing in a region anterior tothe nozzle, means to sup ply air to the passage at a pres-sure aboveatmosphere, and means'to conduct the heated air from the passage to thenozzle. fl

I 2. Apparatus for the drafting of hot gases comprising a conduit forthe flow of the gases to the atmosphere, a nozzle arranged to dishargeair into the conduit to entrain the gases and cause flow thereof in thedirection desired, means providing a passage through which air maytravel in counterflow heat-transfer relationship with the gases flowingin a region anterior to the nozzle, means to supply air to the passageat a pressure above atmosphere and in such a manner as to cause the airto flow therethrough in a direction generally opposite to that of thegases in the conduit, and means to conduct the heated air from thepassage to the nozzle.

3. Apparatus for the drafting of hot gases comprising a conduit for theflow of the gases to the atmosphere, a nozzle arranged to discharge airinto the conduit to entrain the gases and cause flow thereof in thedirection desired, means providing a passage through which air maytravel in counterflow heat-transfer relationship first with the gasesflowing in a region posterior to the nozzle and then with the gasesflowing in a region anterior to the nozzle, means to supply air to thepassage at a pressure above atmospheric and in such a manner as to causeflow therethrough in the said counterflow relationship, and means toconduct the heated air from the passage to the nozzle.

4. Apparatus for the drafting of hot gases comprising an upright stackopen at its upper end to the atmosphere, means to deliver hot gases tothe lower end of the stack, a nozzle arranged to discharge air upwardlyinto the stack and thereby entrain the gases and cause upward flowthereof, a casing surrounding the stack to progases to the lower end ofthe stack, a nozzle mounted within the stack in a position to dischargeair upwardly and thereby entrain the gases and cause upward flowthereof, a casing surrounding the stack to provide an annular passagewhich extends for a substantial distance below the nozzle, means tosupply air at a pressure above atmospheric to the upper end of thepassage for downward flow therethrough in counterflow heat-transferrelationship with the gases flowing upwardly beneath the nozzle, and

'means to conduct the heated air from the lower end of the passage tothe nozzle.

6. Apparatus for the drafting of hot gases comprising an uprightVenturi-shaped stack having a restricted throat and open at its upperend to the atmosphere, means to deliver hot gases to the lower end ofthe stack, a nozzle mounted within the stack in a position to dischargeair upwardly into the throat and thereby entrain the gases and causeupward flow thereof, a casing surrounding the stack to provide anannular passage, the passage extending from a region located asubstantial distance above the nozzle to a region located a substantialdistance below the nozzle, means to supply air at a pressure aboveatmospheric to the upper end of the passage for downward flowtherethrough in counterflow heat-transfer relationship with the gasesflowing upwardly both above and below the nozzle, and means to conductthe heated air from the lower end of the passage to the nozzle.

7. Apparatus for the drafting of hot gases comprising an uprightVenturi-shaped stack having a restricted throat and open at its upperend to the atmosphere, means to..;deliver hot gases to the lower end ofthe stack, a nozzle mounted within the stack in a position to dischargeair upwardly into the throat and thereby entrain the gases and causeupwardflow thereof, a casing surrounding the stack to provide an annularpassage havin a relatively small width radially, the passage extendingfrom a region located a substantial distance above the nozzle to aregion located a substantial distance below the nozzle, an annular inletchamber communicating directly with the upper end of the passage andhaving an outside diameter appreciably exceeding that of the adjacentportion of the passage, means to supply air at a pressure aboveatmospheric to the inlet chamberv for downward flow therefrom throughthe passage in counter-flow heat-transfer relationship with the gasesflowing upwardly both above and below the nozzle, an annular outletpassage communicating with the lower end of the passage and having anoutside diameter appreciably exceeding that of the adjacent portion ofthe passage, and means to conduct the heated air from the outlet chamberto the nozzle.

STRICKLAND KNEASS, JR.

